This invention relates to a sequential blasting system including a plurality of sequentially triggered detonator stages, each having an explosive charge. Blasting systems of this type are specifically used in mining. In a typical example, 100 or more boreholes are drilled into the working face, each hole being filled with an explosive charge together with its associated detonator and being closed by a plug. In order to guarantee efficient demolition, it is important that the charges be fired one after another in a predetermined sequence, with a typical delay time of 30 ms between successive ignitions.
U.S. Pat. No. 4,099,467 describes a sequential blasting system with a plurality of detonator stages to be triggered in succession, wherein each of said detonator stages includes a thyristor and detonator means for detonating at least one explosive charge, said detonator means being connected in series with the output circuit of said semiconductor switch, and the resultant series circuits being connected in parallel between supply leads attached to a power source. The gate of the thyristor is connected to the tap of a voltage divider, which includes the detonator of the preceding stage, respectively. When the preceding detonator is triggered, its resistance changes from an initially low value effectively to infinity, thereby rendering the thyristor of the following stage conductive. The next current pulse activates the series detonator.
Every detonator is connected in parallel with a melting fuse provided to ensure that the change in resistance that is required to trigger the next stage and thus continue to trigger the sequential blasting system occurs even in the event that some location does not have a detonator. This melting fuse, however, constitutes a shunt and as such increases the current requirements considerably.
Another difficulty is that such a melting fuse constitutes an additional, separate component that must be inserted into every detonation stage. If the fuse in a printed circuit is realised by a thin segment of the PCB track, close tolerances must be observed when manufacturing the printed circuit, thus causing a cost increase.
If a detonator is attached, but defective in the sense that, although it fires, it does not do so immediately, but rather when the associated explosive charge becomes highly resistive (typically between 0.5 and 1.5 s later), this results in an exceedingly long delay within the sequential blasting system so that the shock wave at the working face cannot be propagated in the programmed manner. This substantially reduces the reliability of the blasting system, since the interval between the electrical sequence and the blast is shortened considerably.
The sequential blasting system known from U.S. Pat. No. 4,760,791 is plagued by similar problems. In this case, each detonator is connected in parallel with a transistor which conducts current even if a detonator should be missing at this site, thus preventing the blasting sequence from being interrupted by a missing detonator at this location. The parallel circuit consisting of the detonator and transistor is also connected in series with a melting fuse which is intended to prevent a delay in pulse propagation until the time of actual detonation by an improperly functioning detonator, i.e. one that does not become highly resistive immediately. The difficulties described above also exist in the case of the known sequential blasting system.
A far more serious problem is the fact that another transistor is used in such a way that it is shorted when the circuit is functioning correctly in order to produce a short-circuit for the detonation pulse. Since this destroys the transistor, it is not possible to check the known sequential blasting system to ensure that it will function properly before actually being put to use. Likewise, it is not possible to reuse the electronic circuitry of the blasting system. Finally, there is a danger that due to the considerable currents involved, the base solder sites that are not very sturdy to begin with will melt even before the detonator has been triggered.
Moreover, it is necessary to insert a separate activating element at the beginning of the blasting system, thus making it impossible to make the desired length of blasting system simply by cutting off sections or by joining additional sections to the end.
German Auslegeschrift 2,356,875 describes another sequential blasting system in which every detonator stage contains an oscillator, a frequency divider and two driver stages in addition to the actual detonator itself. The triggering pulse that arrives from the preceding detonator stage activates the first driver stage which in turn trips a switch to actuate the oscillator, the frequency divider and the second driver stage. The output of the frequency divider supplies the triggering signal for the next successive detonator stage in the blasting system, while the second driver stage actuates another switch that activates the detonator. Furthermore, every detonator stage also contains a capacitor to store the total energy required for detonation.
In this case, pulse propagation is independent of the presence and proper functioning of the detonator. This, however, necessitates an unreasonable amount of circuitry for practical sequential blasting systems.
German Auslegeschrift 1,287,495 discloses a sequential blasting system with a plurality of detonator stages to be triggered in succession, wherein each detonator stage includes a semiconductor switch and detonator means for detonating at least one explosive charge. The detonator means are connected in series with the output circuit of the semiconductor switch, and the resultant series circuits are connected in parallel between supply leads connected to a power source. The control input of the semiconductor switch of each detonator stage is connected to the junction between the semiconductor switch and the detonator means of the respective preceding detonator stage.
In this system, the propagation of the control signal from one detonator stage to the next is effected solely by the change in the switching stage of the semiconductor switch. This means that the sequential blasting system will remain functional even if individual detonators are missing or do not become highly resistive as they should. Since every semiconductor switch can become conductive and can activate the associated detonator means, when the semiconductor switch of the respective preceding stage has been triggered, the predetermined blasting sequence will be necessarily observed. The fact that the blasting system has been improperly assembled cannot cause the system to start detonating at two different locations simultaneously when the power source is switched on.
The known blasting system, however, requires at least one capacitor in each detonator stage to pass the triggering pulse from one detonator stage to the next. Due to the presence of such capacitors, the known circuit may not be fully integrated.
Moreover, this circuit again requires a separate circuit element connected to the input of the first detonator stage in order to initiate the blasting sequence, so that the system may not be elongated or shortened as desired--at least not at the end of the first detonator stage.